To me the choice was not a easy one, made even harder by the fact that you are considering a PCI Express model as the PCI-E 6600GT is pretty low priced when compared to the 6800. I was purchasing AGP so it was easier for me as the AGP 6600 is closer to the 6800. I chose the 6800 because of the 256 bit path, the better performance, and the chance to unlock it, plus I wanted to go longer without having to upgrade again.
Also I was sucessful with unlocking the extra pipelines and the extra shaders. So now my card is 16 Pipes 6 Shaders and it runs perfect. Of course even if I had not I would of still been happy.
If you do go with the 6800 dont count on it unlocking though as I think its close to a 50/50 chance (Well I hear 60% are sucessful). If you do purchase one, purchase it for what it is and you may get lucky and get something better, but again dont purchase it expecting it to unlock as you may be disappointed.